1. Field of the Invention
The invention relates to rotary transformers suitable for use in applications such as video, audio and data tape recording devices wherein multiple recording heads on a rotating drum must be electrically coupled to fixed electronic circuitry.
2. Description of the Related Art
Certain types of recording technology require electrical coupling between a rotating drum, carrying a plurality of reading and recording heads, to fixed electronic circuitry which interprets signals read by the heads and which controls recording by supplying current to the heads. The circuitry is generally provided at some point adjacent to the rotating drum, with signals read from the recording medium and signals for recording (or writing) information to the storage medium being transferred to and from the rotating drum by means of a rotary transformer.
Typically the rotary transformer is comprised of two circular plates, each with a plurality of windings. One of the plates and its associated windings comprises the transformer primary while the other comprises the transformer secondary. Each pair of windings may comprise a read channel, a write channel, or both. The transformer is designed with the goal of providing isolation between the respective channels during coupling of the electrical signals from one plate/winding to the opposing plate by magnetic inductance so that channel cross-talk is minimized.
A typical rotary transformer is shown in FIGS. 1A and 1B. FIGS. 1A and 1B are reproductions of two drawings shown in FIG. 1 in H. Inoue and Y. Okada, Finite Element Analysis of Crosstalk in a Rotary Transformer for VCR's, I.E.E.E. Transactions On Magnetics, Vol. 27, No. 5, September 1991, pp. 3931-3934. As shown therein, a conventional transformer 40 includes a first, rotating plate 20 positioned parallel to and coincident with a second, stationary plate 30. Stationary plate 30 is separated from rotating plate 20 by an air gap A of about 3 milli-inches. Each plate 20,30 includes five grooves: 22, 24, 25, 26, 28, on plate 20; and 32, 34, 35, 36, 38 on plate 30. Windings 21, 23, 27, 29 and windings 31, 33, 37, 39, are respectively provided in grooves 22, 24, 26, 28 and 32, 34, 36, 38. In alternative embodiments, central grooves 25, 35, would also include a winding, but in the embodiment shown in FIGS. 1A and 1B, these central grooves 25,35 are left without windings to enhance channel isolation, as discussed herein. Plates 20 and 30 may be typically manufactured from ferrite, such as Ni-Zn ferrite.
Four separate electrical channels are provided by rotary transformer 40. Each channel is defined by a pair of windings as follows: channel 1, windings 21 and 31; channel 2, windings 23 and 33; channel 3, windings 27 and 37; and channel 4, windings 29 and 39. Typically, fixed-side windings 31, 33, 37, 39 have six turns each, while rotating side windings 21, 23, 27, and 29 having three turns each. As will be understood by one skilled in the art, when an alternating current source is coupled to one of fixed-side windings 31, 33, 37, 39, inductive coupling occurs between the winding and the corresponding rotating side windings 21, 23, 27, and 29. The same is true when a signal is induced by the recorded signal from the storage medium and supplied to the secondary windings for transfer to the primary windings.
In all systems utilizing multi-channel rotary transformers, it is desirable to reduce cross-talk between the respective channels and provide good channel isolation so that signals on one particular channel will not be imparted to another channel. In a video recorder, for example, four heads--each coupled to a single transformer channel--are used to scan over the surface of a helically wound tape over the rotating surface of a cylindrical drum. (This is typically known as "helical scan" tape recording.) In a helical scan system, each of the four heads will be separated on the drum at equidistant points, 90.degree. apart. As such, while only one head is dedicated to recording information at any particular point in time, usually more than one head will be in contact with the tape. If any cross-talk between the transformer coils is present, an undesired signal may be imparted to one or more heads in contact with the tape at the same time as the read or write dedicated head. Some such systems have incorporated the use of a multi-channel pre-amplifier circuit to eliminate cross-talk; however such circuits increase the complexity and the cost of the overall application in which they are used.
Thus, channel windings are deposited in grooves 22, 24, 26, 28, and 32, 34, 36, and 38 in each plate to improve isolation between the channels. A minimal amount of ferrite is disposed between the windings. As the diameter of the elements becomes smaller, the winding-to-winding spacing, and hence the width of the ferrite between the grooves, decreases. In some designs, alternating grooves will be left vacant to increase isolation between the channels by increasing the flux path which must be traversed to establish any coupling. Yet another common alternative, (as described in Finite Element Analysis of Crosstalk in a Rotary Transformer for VCR's), is to provide a shorting ring 42 between the respective windings in a vacant groove, in this case central groove 25, to improve isolation between the channels.
An additional concern in rotary transformers is the commutative efficiency level of each of the channels. As noted above, windings on the rotating element have fewer turns than those on the stationary element in accordance with the well known principle that the ratio of turns in transformer winding is proportional to the voltage induced in the primary or secondary windings n.sub.s /n.sub.p =V.sub.s /V.sub.p. In conventional rotary transformers, all turns being equal from channel-to-channel, there is greater inductance between channels with greater area--those at the outermost diameter of the plates. In a number of rotary transformer applications, particularly applications involving data storage and retrieval, uniformity in channel-to-channel performance is highly desirable. While channel-to-channel uniformity may sometimes be achieved through modification of the pre-amplifier circuitry, this solution has limited effectiveness. Further, mechanical solutions, such as increasing the turns per winding at the smaller diameter of windings, have been more problematic. Specifically, limited winding area is available at smaller diameter grooves, especially if a design goal is to limit the overall size of the transformer.
Such conventional rotary transformer designs are also inherently difficult to manufacture. First, the magnetic core material must be either cast as an integral piece with grooves, or the grooves machined from a single piece of core material. Machining of rotary transformers having a total edge to edge diameter on the order of less than one inch is problematic and costly. After machining, the wires for the windings must be circumferentially wound in the grooves. When a large number of transformer channels (and hence grooves) are required, the process naturally becomes more difficult and time consuming.
Miniaturization of the conventional rotary transformer design is also problematic. The dimensions of the multichannel rotary transformer are limited by manufacturing tolerances and the cross-sectional area of the core material between the windings. In addition, a minimal amount of winding area is necessary to provide effective read voltage coupling in applications such as VCR's and tape data storage units.
As noted above, rotary transformers find application in tape drive data storage units. In such applications, channel-to-channel efficiency, transformer manufacturability, and optimal channel performance are significant factors in improving overall drive performance and reducing the total cost of manufacture. U.S. patent application Ser. No. 07/898,926, entitled Arcuate Scanning Tape Drive, filed Jun. 12, 1992, by J. Lemke (hereafter "the Lemke application") discloses a relatively compact tape drive for recording and playing back a data tape having a storage capacity of approximately 10 gigabytes, which storage capacity is higher than that previously obtained with either longitudinal or helical recording. FIGS. 2A, 2B, 3A, 3B and 3C are reproductions of FIGS. 1, 2, 7, 8A and 8B, respectively, of the Lemke application. The Lemke application discloses a tape drive 110 including a plurality of heads 135 placed on the front circular face of a rotating drum 130, with the axis of rotation 138 of the rotating drum 130 being perpendicular to and intersecting with the longitudinal axis of the advancing tape 118. As the tape 118 advances from the right to the left (in FIG. 2b) and the drum rotates in a counterclockwise direction, the heads 135 trace arcuately-shaped data tracks substantially transverse to the longitudinal axis of the tape. Arcuate scan recording has been known for some time, but has been disfavored due to the lack of effective servoing schemes for accurately maintaining alignment of the heads with the arcuate data tracks. Another reason arcuate scan recording has been disfavored is that existing head/tape engagement mechanisms employed in arcuate scan tape drives, such as a backing plate for urging the tape against the head, often caused damage to the heads and/or tape in a relatively short period of time.
The head mechanism in the Lemke application includes at least one read, write and servoing head 135 mounted on the front face of the rotating drum. The drive further includes a dual adjustment servoing scheme to maintain head/track alignment in response to servo feedback signals.
In arcuate scan tape drives, such as those disclosed in the Lemke and instant applications, it is also difficult to provide effective coupling of the heads to the control circuitry. It is imperative that performance of the rotary transformer in such drives be optimal for each head channel and that each head channel have equivalent performance characteristics. The transformer scheme disclosed in the Lemke application is shown in FIG. 3A.
FIG. 3A is a perspective, cutaway view showing the transduction architecture of the rotary head assembly 130 of the arcuate scan drive disclosed in the Lemke application. In the exemplary 3-head architecture of the embodiment illustrated in FIG. 3A, the read, write, and servo head assemblies are arranged in counterclockwise sequence on the side surface of a transducer drum 200. The three heads are substantially identically constructed in all essential respects except for the width and radial position on the drum face. Each head assembly includes a transducer support block, such as the read support block 202b, attached to the side cylindrical surface of the transducer drum by a threaded screw, such as screw 202c. Each transducer support block carries a two winding transducer. The read transducer is indicated by reference numeral 202a, the write transducer by reference numeral 204a, and the servo transducer by reference numeral 206a. In practice, drum 200 is slotted along its side to accept the transducer support block so that the transducers are positioned near the outer periphery of the face of each drum.
The Lemke application notes that it would be possible to serve all three of the transducers in FIG. 3A by single channel rotary transformer having a rotor mounted winding and a stator mounted winding. However, as noted therein, this would necessitate the provision of switched electronics to effectively provide separate read, write, and servo channels. However, the combination of impedance effects exhibited by a single multi-functional rotary transformer and the artifacts of electronic switching would increase channel noise. Accordingly, separate, dedicated rotary transformer is utilized for each head. A read head rotary transformer consisting of a rotor mounted winding/core piece 212 and a stator mounted winding/core piece 222 is shown. The rotor piece 212 is connected to the read transducer 202a by a twin lead signal path 212a. The write head 204 is served by a rotary transformer consisting of a write rotor-mounted winding/core piece 214 and a stator-mounted winding/core piece 224. The rotor piece 214 is connected to the right transducer 204a by a twin-lead signal path 214a. Lastly, the servo head is connected by a twin-lead signal path 216a to a servo rotor-mounted winding/core piece 216 which operates in conjunction with a servo stator-mounted winding/core piece 226.
Stator pieces 222, 224 and 226 are mounted in a fixed manner on a portion (not shown) of the rotor head assembly 160. Each stator piece includes a substantially arcuately-shaped core section, which corresponds with the shape of the tracks produced and read back by the apparatus described in the Lemke application. The transducer drum 200 is mounted for counterclockwise rotation on shaft 208. In this embodiment, the rotary transformers are essentially equivalently constructed. In this regard, both the rotor and stator pieces are quadripole devices whose electromagnetic operation cancels any effects caused by external fields. Each transformer consists of a rotor and stator piece located at essentially the same radial distance but at different circumferential location as its companion piece. It is contemplated that the functions performed by these transformers could be accomplished with a set of coaxial circular transformers, one for each head. However, the cost for the configuration would be higher than the cost for the configuration illustrated in FIG. 3A.
The use of separate dedicated rotary transformers with stationary windings necessitates the positional relationships illustrated in FIGS. 3B and 3C. (Note that in FIG. 3A, the winding/core pieces are illustrated and described; however, FIGS. 3B-3C illustrate the relative core positions.) As shown, the three stator pieces 222, 224, and 226 are disposed to occupy arcuate sections of a single circular plane which is parallel to, and concentric with, respective circular planes occupied by the rotor pieces 212, 214 and 216, and the transducers 202a, 204a, and 206a. The orientation of these components with respect to a tape at the record/playback location is illustrated in FIG. 3B. In FIG. 3B, a tape 230 having an upper edge 231 and a lower edge 232 is transported from right to left, while the transducer drum with the transducers and rotary windings disposed thereon rotates in a counterclockwise direction at a center of rotation 138. Ideally, the center of rotation 138 falls on the center line 234 of the tape 230 during record/readback. The stator pieces 222,224, and 226 are spaced arcuately from each other by distances adequate to ensure that they are not bridged by the cores of the rotary pieces. The stator pieces are permanently positioned such that the upper left corner of the stator piece 224 is substantially aligned with the upper edge 231 of the tape. The arcuate span of the write stator piece 224 extends from the upper edge 231 across the lower edge 232 of the tape 230. Assume now that as the transducer drum 200 rotates counterclockwise, the write transducer 204a and write rotor piece 214 have the positions illustrated in FIG. 3B. Application of a write current to the winding of stator piece 224 will generate a field which is coupled to the rotor piece 214, causing current to be conducted therefrom to the transducer 204a, enabling the write transducer to record an arcuately shaped track on the tape 230.
When the transducer drum has rotated in a counterclockwise direction far enough to enable the write transducer 204a to record a full track, the read transducer 202a must be positioned to begin reading the track just written in order to support read-after-write validation. This is shown in FIG. 3C. However, the physical channelization of the write and read functions in the rotary transformer prevents the use of the write stator winding 224 to couple the readback signal. Instead, this is provided in the read stator winding 222. As FIGS. 3B and 3C illustrate, the read stator winding 222 is displaced in the forward arcuate scanning direction from the write stator winding 224. Further, the read rotor winding 212 is located on the transducer drum forward of the write rotary winding 214 in the scanning direction by an arcuate distance sufficient to align it with the read stator winding 222 when the read transducer 202a is positioned at the beginning of a track.
The transformer disclosed in the Lemke application requires three separate rotor coils and three separate stator coils (e.g., two coils per channel) for the particular arcuate scan head assembly shown therein. The transformer scheme requires precise positioning of the rotor and stators for signal coupling, and, according to the application, sacrifices manufacturability for performance. In addition, the size of the transformer scheme in the Lemke application is difficult to reduce, making the system cumbersome in the small confines of the form factors generally associated with tape data storage units. In addition, this scheme makes it difficult to alter the number of channels since a minimum physical area is required for each channel to ensure signal coupling.